1. Field of the Invention
The invention relates to the field of signal processing, particularly audio signal processing.
2. Prior Art
Numerous systems are commercially available and otherwise known for processing audio signals prior to transmission or recording. In most cases, these systems are used to reduce the peak-to-r.m.s. ratio of audio signals. A lower ratio provides greater loudness and coverage for given limitations of carrier power. Simultaneously, it is desirable to avoid the audible side effects associated with more conventional audio signal processing. The prior art section of U.S. Pat. No. 4,208,548 discusses these side effects and other limitations of prior art processing methods.
U.S. Pat. No. 4,208,548 describes a variable-gain (VCA) amplifier used in conjunction with a clipper. Complex psychoacoustical calculations are described for controlling the gain of the VCA to prevent the clipper distortion from being detectable. Unnatural gain and loudness variations sometimes result in this system because the VCA gain needed to reduce clipping distortion to an undetectable level is so small that the ear perceives the loudness as being reduced in an unnatural way. In more conventional processing, this phenomena is sometimes referred to as "pumping". As will be seen, the present invention provides a substantial improvement over the processing techniques described in the abovementioned patent.
It is well-known that multiband compressors can cure many of the problems associated with wideband compressors. This topic is addressed in U.S. Pat. No. 4,249,042. There have been some unsuccessful attempts to employ multiband clipping in addition to multiband compression. And as a result of these attempts, it is known that if this clipping is done in an obvious way (one clipper after each compressor in a multiband compressor) that distortion builds up even faster than it does when a single clipper is used. Thus, for the most part, the prior art has used multiband compression without clipping or systems employing wideband clipping.
The present invention, as will be discussed, provides an apparatus for multiband clipping which results in substantially lower perceived distortion than a system employing a single clipper. In practice, the output of the invented processor (which includes multiband compression) can be directly applied to a final wideband clipper on virtually all program material without objectionable distortion. The more complex psychoacoustical calculations described in U.S. Pat. No. 4,208,548 are eliminated. The invented system provides improved naturalness and greater simplicity (at a lower cost). The new multiband processing of the present invention uses an extension of the distortion-cancellation apparatus disclosed in U.S. Pat. No. 4,208,548. Additionally, the present invention extends the concept described in U.S. Pat. No. 4,249,042, in particular, FIG. 4 and the circuitry associated with clipper 71.
The present invention makes use of a distributed allpass crossover method. Some theories on such a system are described in "All-Pass Crossover Systems" Journal of the Audio Engineering Society, September, 1980, Vol. 28, No. 9, beginning at page 575, by Peter Garde.